Method for controlling a nuclear fuel pencil

ABSTRACT

An electromagnetic induction detector ( 9 ) is used to check the weld ( 6 ) of a plug ( 3 ) to a nuclear fuel cladding ( 2 ) still containing a spring ( 5 ). A weld made using an excessive energy weakens the metallurgical structure of the spring ( 5 ) which may reduce the electromagnetic coupling and the signal recorded by the detector ( 9 ). A comparison between the peak values of the signal and the plateau value of the signal gives a criterion for estimating the quality of the fuel rods ( 1 ).

The purpose of this invention is a method for inspecting a nuclear fuelrod.

Some nuclear fuel rods comprise a cladding containing a stack of fuelpellets closed at one open end by a plug that is subsequently welded. Aspring is compressed between the plug and the stack of pellets to holdthe stack in place.

Non-destructive tests are carried out to check some characteristics ofthe assembled rod. Thus, an electromagnetic detector is used to checkthat the spring is present and the length of the chamber that containsit between the plug and the fuel pellets. The rod is moved in front ofthe detector, and the detector output signal, which is stronger when themagnetic coupling is locally higher, includes a plateau corresponding tothe length of the spring and is limited by two peaks corresponding tothe ends of the spring, in which the compressed turns are thereforeaccumulated and the magnetic coupling is greater. The existence of thischaracteristic curve proves that the spring has not been forgotten.

The process in question here uses a measurement of this type todetermine the quality of the plug weld, based on an innovativereasoning.

It has been observed that the weld quality is strongly dependent on theinput energy. If the input energy is excessive, molten metal dropletscan be deposited on the spring and affect its state or make it stick tothe plug. One particular problem that could arise is that themetallurgical structure of the spring, the cladding or the plug could bemodified due to overheating. A rod with a damaged spring must berejected, in the same way as a rod with a missing spring.

To summarise, the invention relates to a method for the inspection of anuclear fuel rod with an end plug welded to a cladding, a coil springbeing compressed between the plug and the fuel, using an electromagneticinduction detector along which the rod is moved, by measuring a detectoroutput signal, characterised in that it consists of recording a value ofthe signal at a base portion, obtained at the side of the rod and in theabsence of any magnetic coupling with the rod, and another value of thesignal and with a peak adjacent to the base portion, where the plug andone end of the spring adjacent to the plug apply maximum magneticcoupling, calculating a ratio of the values of the signal and acceptingor rejecting the rod depending on whether the ratio is more than or lessthan a threshold.

The invention will now be described with reference to the followingfigures:

FIG. 1 is a general view of the equipment used for implementation of theprocess,

FIG. 2 illustrates the curve obtained.

FIG. 1 shows a rod 1 made of nuclear fuel for which the cladding ismarked as reference 2, the top plug is reference 3, the stacked pelletsare reference 4, and the spring is reference 5. The weld of the plug 3to the edge of the cladding 1 is marked reference 6. It extends adjacentto one end 7 of the spring 5, and the other end adjacent to the pellets4 is marked as reference 8. The inspection is made by a sensor 9comprising a cylindrical mandrel 10 in which a hole 11 is formed for thepassage of the rods 1 and in which two successive outer grooves areformed, into one of which a primary coil is fitted and into the other ofwhich a secondary coil 13 is fitted. An excitation device 14 is placedat the terminals of the coil 12 and a reception device 15 is placed atthe terminals of coil 13. Each of the coils 12 and 13 may comprise 530turns of copper wire with a diameter of one tenth of a millimetre. Asinusoidal electrical signal output from an oscillator is input into theprimary coil 12; the frequency of the oscillator may be 1.5 kilohertzand its voltage 2 volts. The secondary coil 13 carries an inducedcurrent for which the voltage depends on the magnetic coupling producedby the rod 1. The rod 1 is moved along the detector 9 by passing throughthe recess 11, to result in a signal with a complex shape shown in FIG.2. The abscissas express a movement distance and the ordinates a voltagemeasured by the secondary coil 13. Firstly, a constant value or a basevalue is recorded corresponding to a lack of magnetic influence of therod 1, or a no-load test, for which the value S1 is about 2.6 volts inthis example. The value of the signal increases suddenly when themagnetic coupling produced mainly by the end 7 of the spring 5 is felt,up to a peak value S4 reaching about 4.0 and 4.5 volts in this example.The magnetic coupling is then produced mainly by the central portion ofthe spring 5, such that the signal is equal to a lower value of theplateau S3, equal to about 3.3 or 3.4 volts. When the magnetic couplingis produced mainly by the other end 8 of the spring 5, the signalreturns approximately to a high value of the peak S5 that is in practiceclose to S4, or is sometimes a little higher. Finally, if the rod 1 ismoved further through the detector 9, the signal becomes equal to astable value S2, coupling being produced essentially by the cladding 2,and for which the value is slightly higher than S1.

The process according to the invention consists of recording values S4,S5 and S3 and determining the ratio$\frac{( {{S4} - {S5}} )}{S3}.$It has been observed that if excess energy is used to weld the plug 3,the value of S4 is lower. Therefore the inspection process consists ofcomparing the value of the ratio$\frac{( {{S4} - {S5}} )}{S3}$with a threshold, that may be −10% in the example considered but must bedetermined empirically in other cases, and to appraise whether or notthe rods 1 comply with the manufacturing requirements, depending onwhether this ratio is greater than or less than the threshold (in theexample, less than the threshold). This curve is also used to determinethe compressed length of spring 5 or the length of the chamber thatcontains it, by measuring the distance between the two peaks, and thepresence of the spring 5, since no peak is observed if it is missing,and the measurement curve changes directly from the value S1 to thevalue S2.

The choice of the ratio $\frac{( {{S4} - {S5}} )}{S3}$as the acceptance criterion is based on the fact that the values of thepeaks S4 and S5 do not depend only on the magnetic coupling of the rod 1at the location sensitive to manufacturing defects, but alsocharacteristics of the environment, the spring length and the detectoritself. Therefore, it is better to weight these values S4 and S5 by theplateau value S3 that gives a sort of reference signal.

1. Method for the inspection of a nuclear fuel rod (1) with an end plug(3) welded to a cladding (2), a coil spring (5) being compressed betweenthe plug (3) and the fuel (4), using an electromagnetic inductiondetector (9) along which the rod is moved, by measuring a detectoroutput signal, characterised in that it consists of recording a value ofthe signal at a base portion (S3), obtained with non-adjacent springcoils, and other values of the signal at peaks (S4 and S5) adjacent tothe plateau portion, and which apply maximum magnetic coupling,calculating a ratio $\frac{( {{S4} - {S5}} )}{S3}$ of thevalues of the signal and accepting or rejecting the rod depending onwhether the ratio is more than or less than a threshold.